JP2015102584A - Image forming apparatus - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide an image forming apparatus configured to transfer a toner image on a latent image carrier to an intermediate transfer body by constant current control, while preventing toner for imaging an upstream toner image from being excessively consumed due to increase in reverse transfer rate in a downstream transfer position.SOLUTION: In a printer 100 having a plurality of photoreceptors 1, a black primary transfer roller 17K controls a transfer voltage by constant current control. A magenta transfer nip downstream optical sensor 152M and an optical sensor 151 are provided for detecting changes in the amount of toner adhered before and after a magenta grayscale pattern image Sm or the like is transferred to an intermediate transfer belt 6 by means of a magenta primary transfer roller 17M in upstream of the black transfer nip 60K passes through a black transfer nip 60K. The black primary transfer roller 17K changes a target current value in the constant current control, on the basis of a detected change in the amount of toner adhered.

Description

  The present invention relates to an image forming apparatus such as a copying machine, a facsimile, and a printer.

In this type of image forming apparatus, a latent image carrier made of a photosensitive member or the like is rotated and the latent image carrier is uniformly charged by a charging device, and then the latent image carrier is exposed to light by an exposure device. To form an electrostatic latent image. The toner image on the latent image carrier formed by developing the electrostatic latent image with a developing device is transferred to a recording medium such as transfer paper by a transfer device.
In an image forming apparatus that performs color image formation, a plurality of latent image carriers on which a single color toner image of each color is formed are arranged opposite to an intermediate transfer body such as an intermediate transfer belt along the surface movement direction. A so-called tandem type image forming apparatus is known. The tandem type image forming apparatus includes a plurality of image forming devices that form a single color toner image on each of the plurality of latent image carriers, and each image forming device places a single color toner image on each latent image carrier. Form. The toner images formed on the plurality of latent image carriers are sequentially primary-transferred to the intermediate transfer body at respective primary transfer nips.
In such an image forming apparatus, for example, the developing ability changes due to a change in environment or an error factor such as deterioration of the apparatus over time, and even if an image is formed under the same image forming conditions, the image density varies over time. End up.

  In order to solve such a problem, it is possible to perform a process called process control for forming a gradation pattern image with a changed toner adhesion amount per unit area and determining an image forming condition for obtaining an appropriate image density. Known (Patent Documents 1 and 2, etc.). As process control, an electrostatic latent image of a patch pattern image as a reference pattern image is formed on a latent image carrier, developed with a developing device, transferred to an intermediate transfer member, and then transferred onto the intermediate transfer member. The developing ability of the developing device is calculated from the toner adhesion amount per unit area of the patch pattern image. Then, various potentials such as a developing bias and a charging potential of the latent image carrier are determined from the developing ability. By performing such process control, stable image formation can be performed even if the installation environment changes or deterioration with time occurs. Hereinafter, the toner adhesion amount per unit area is simply referred to as “toner adhesion amount”.

  In general, the primary transfer bias applied to the intermediate transfer member when the toner image is primarily transferred from the latent image carrier to the intermediate transfer member is biased by constant current control. In the configuration in which the primary transfer bias is controlled at a constant current, the primary transfer voltage varies depending on the surface potential of the latent image carrier.

  In a tandem type image forming apparatus, a toner image (hereinafter referred to as “upstream toner image”) formed by an upstream image forming apparatus and transferred to an intermediate transfer member is a primary transfer of a downstream image forming apparatus. It passes through a nip (hereinafter referred to as a “downstream transfer nip”). When the upstream toner image passes through the downstream transfer nip, reverse transfer occurs in which part of the toner forming the upstream toner image moves to the latent image carrier that contacts the intermediate transfer belt at the downstream transfer nip. . This reverse transfer tends to increase as the primary transfer voltage at the downstream transfer nip increases. When the downstream primary transfer voltage fluctuates, the amount of toner transferred reversely with respect to the toner amount of the upstream toner image entering the downstream transfer nip The ratio (hereinafter referred to as “reverse transcription rate”) also varies.

Further, as a result of extensive studies by the present inventors, it has been found that the reverse transfer rate in the downstream transfer nip also varies depending on the charge amount (Q / M) of the toner forming the upstream toner image. . Specifically, it has been found that the reverse transfer rate decreases as the toner charge amount of the upstream toner image increases, and the reverse transfer rate increases as the toner charge amount of the upstream toner image decreases.
Since the reverse transfer rate fluctuates due to the influence of various factors, it is difficult to predict the amount of change in the reverse transfer rate even if a change in the element affecting reverse transfer is detected.

  In the conventional tandem type image forming apparatus, the position of detecting the toner adhesion amount per unit area of the patch pattern image on the intermediate transfer member is the downstream side of the primary transfer nip on the most downstream side in the process control. The surface of the intermediate transfer member. In the upstream toner image, the toner adhesion amount after passing through the downstream transfer nip, in other words, the toner adhesion amount after the adhesion amount is reduced by reverse transfer is detected. In such process control, since the image forming conditions are controlled so that the toner adhesion amount after the adhesion amount decreases becomes the desired toner adhesion amount, even if reverse transfer occurs, the desired toner adhesion amount is reduced. A toner image can be obtained. For this reason, even if the reverse transfer rate at the downstream transfer nip is high, the effect does not appear in the final image, and the state where the reverse transfer rate is high cannot be detected.

However, if process control is performed while the reverse transfer rate at the downstream transfer nip is high, the image forming condition for forming the upstream toner image is that much toner is reversely transferred at the downstream transfer nip. Is also determined so as to obtain a desired toner adhesion amount. If image formation is continued under the image forming conditions determined in this way, most of the toner that forms the upstream toner image continues to be consumed by reverse transfer at the downstream transfer nip. For this reason, if the image forming conditions are determined and image formation is performed in a state where the reverse transfer rate in the downstream transfer nip is increased, the toner for forming the upstream toner image is continuously consumed.

  The present invention has been made in view of the above problems, and the object thereof is as follows. That is, the toner image on the latent image carrier is transferred to the intermediate transfer member by constant current control, and the toner that forms the upstream toner image is caused by the increase in the reverse transfer rate at the downstream transfer position. An object of the present invention is to provide an image forming apparatus capable of suppressing excessive consumption.

  In order to achieve the above object, the invention of claim 1 of the present application provides a plurality of images forming toner images based on image information on the surfaces of a plurality of latent image carriers that move on the surface, and the latent image carriers. An image forming apparatus comprising: a plurality of transfer units that respectively transfer a toner image on a body to an intermediate transfer member that moves on the surface; and a recording medium transfer unit that transfers the toner image on the intermediate transfer member to a recording medium. Among the plurality of transfer means, the downstream transfer means having at least one other transfer means on the upstream side in the surface movement direction of the intermediate transfer body than the transfer means controls the transfer voltage by constant current control, The upstream side of the upstream toner image transferred to the intermediate transfer member by the other transfer unit upstream of the downstream transfer position where the downstream transfer unit performs transfer passes before and after passing through the downstream transfer position. Toner image toner A toner adhesion amount fluctuation detecting unit for detecting the fluctuation of the adhesion amount, and the downstream transfer unit is configured to change a target current value in the constant current control based on a detection result of the toner adhesion amount fluctuation detection unit. It is a feature.

  According to the present invention, the toner image on the latent image carrier is transferred to the intermediate transfer member with constant current control, and an upstream toner image is formed due to an increase in the reverse transfer rate at the downstream transfer position. There is an excellent effect that it is possible to suppress excessive consumption of toner to be imaged.

FIG. 4 is an enlarged explanatory view of four primary transfer nips of the printer according to the embodiment and detection positions by an optical sensor. 1 is a schematic configuration diagram of a printer according to an embodiment. FIG. 2 is a block diagram illustrating an example of a main configuration of a control system that performs density adjustment in the printer of the embodiment. FIG. 3 is an explanatory diagram of a first density adjustment pattern image formed on an intermediate transfer belt. FIG. 3 is an enlarged explanatory view in the vicinity of a magenta transfer nip and a black transfer nip. FIG. 5 is an explanatory diagram showing a change in primary transfer voltage due to a change in surface potential of a photoreceptor in constant current control. 6 is a graph showing the relationship between fluctuations in the primary transfer voltage of the downstream transfer nip and fluctuations in the reverse transfer rate of the upstream toner image. FIG. 3 is an explanatory diagram showing a change in primary transfer voltage in constant current control due to a change in layer thickness of the surface layer of the photoreceptor in constant current control. Explanatory drawing of the vicinity of four primary transfer nips when a monochrome image is formed. FIG. 6 is an explanatory diagram illustrating transfer current value control based on a calculation result of a reverse transfer rate of a downstream transfer nip. The graph which shows the relationship between the transfer voltage of a downstream transfer nip, and a transfer rate.

Hereinafter, embodiments of the present invention will be described with reference to the drawings.
FIG. 2 is a schematic configuration diagram showing a configuration example of a tandem intermediate transfer type full-color image forming apparatus (hereinafter referred to as “printer 100”) according to an embodiment of the present invention.

  The printer 100 includes an intermediate transfer belt 6 that is an endless belt-like intermediate transfer member as an image carrier that can be rotationally driven by a drive motor (not shown) as a drive source. The intermediate transfer belt 6 has a multilayer structure. The base layer of the intermediate transfer belt 6 is formed of, for example, a fluororesin that is less stretched, a PVDF (polyvinylidene fluoride) sheet, or a polyimide resin, and the surface thereof is covered with a smooth coating layer such as a fluororesin. . The intermediate transfer belt 6 is wound around support rollers (15, 16, 18, 19) as a plurality of support members, and is configured to rotate counterclockwise (in the direction of the arrow in the figure). . An intermediate transfer belt cleaning device 7 as a cleaning unit for removing residual toner remaining on the intermediate transfer belt 6 after image transfer is provided on the left side of the fourth support roller 19 as one of the support rollers. Yes.

Further, four positions along the surface moving direction of the intermediate transfer belt 6 are provided at positions facing the stretched surface of the intermediate transfer belt 6 stretched between the second support roller 16 and the third support roller 18. The image devices 10 are arranged side by side to constitute a tandem type image forming unit 200. The four image forming apparatuses 10 (Y, C, M, K) are arranged in the yellow (Y), cyan (C), magenta (M), and black (K) colors along the surface movement direction of the intermediate transfer belt 6. Is an image forming means corresponding to.
Below the tandem image forming unit 200, there is provided an optical writing R device (exposure device 5) capable of emitting a laser beam L modulated based on image data.

  The four image forming apparatuses 10 (Y, C, M, and K) have the same structure, and each of the photoreceptors 1 (Y, C, M, and K) serving as latent image carriers on which toner images of the respective colors are formed. ). The yellow image forming device 10Y includes a charging device 2 that is driven to rotate in the clockwise direction in the drawing and charges the surface of the yellow photoconductor 1Y around the yellow photoconductor 1Y. Further, a developing device 3 is provided for developing the latent image formed on the yellow photoconductor 1Y by the laser light L from the exposure device 5. Further, a cleaning device 4 is provided for removing toner as a developer remaining on the yellow photoreceptor 1Y. In the developing device 3, toner is carried on a developing roller (not shown), and the toner on the developing roller is caused by a developing potential that is a potential difference between the developing bias potential applied to the developing roller and the surface potential of the yellow photoreceptor 1 </ b> Y. Supplied to the surface of the yellow photoreceptor 1Y.

  Further, as a primary transfer member for transferring the toner image on the yellow photoconductor 1Y to the intermediate transfer belt 6 at a position facing the yellow photoconductor 1Y with the stretched portion of the intermediate transfer belt 6 interposed therebetween. Primary transfer roller 17 is disposed. The other three color image forming apparatuses 10 (C, M, and K) are configured in the same manner as the yellow image forming apparatus 10Y described above.

A secondary transfer device 12 is disposed at a position facing the portion of the intermediate transfer belt 6 supported by the first support roller 15. The secondary transfer device 12 uses, as a transfer target (recording medium), a composite color image on the intermediate transfer belt 6 formed by primarily transferring a toner image from each photoconductor 1 (Y, C, M, K). A transfer portion for transferring to the paper P is configured. The secondary transfer device 12 of the printer 100 includes a secondary transfer roller 8 as a secondary transfer member, and a pressure member 9 including a spring or the like that presses the secondary transfer roller 8 toward the first support roller 15. ing.
An optical sensor 151 for detecting the toner adhesion amount of the toner image adhered on the intermediate transfer belt 6 is disposed at a position facing the upper portion of the portion of the intermediate transfer belt 6 supported by the first support roller 15. ing.

Above the secondary transfer device 12, a fixing device 13 for fixing a transfer image (superimposed color toner image) on the paper P is provided. The fixing device 13 has a configuration in which a pressure roller 13b is pressed against the fixing roller 13a to form a fixing nip at a contact facing portion of both rollers (13a, 13b). Then, fixing is performed by applying heat and pressure to the toner image on the paper P in the fixing nip.
The secondary transfer device 12 also has a transport function for transporting the paper P after the secondary transfer to the fixing device 13, and serves as a transport guide member between the secondary transfer roller 8 and the fixing nip of the fixing device 13. A transfer outlet guide member 31 and a pre-fixing guide member 11 are provided. The sheet P after the secondary transfer is guided by the transfer outlet guide member 31 and the pre-fixing guide member 11 and is conveyed so as to move toward the fixing nip.

  The transfer medium supply means for supplying the paper P toward the secondary transfer device 12 includes a paper feed table 50, a manual feed tray (not shown), a registration roller pair 28, a transport roller pair 30, a first paper feed path 29a, and a first paper feed path 29a. Two paper feed paths 29b are provided. The paper feed table 50 includes a paper feed cassette 26 and a paper feed roller 27 as a paper supply member that feeds and supplies the paper P on the paper feed cassette 26.

  For example, color image formation in the printer 100 is performed as follows. When a start switch (not shown) of an operation unit (not shown) is pressed, one of four support rollers (15, 16, 18, 19) is rotationally driven by a drive motor (not shown) as a drive source, and the other support rollers are driven. Rotate. Thereby, the intermediate transfer belt 6 is rotated and conveyed in a predetermined rotation direction indicated by an arrow in FIG. At the same time, the photoreceptor 1 (Y, C, M, K) is rotationally driven in each image forming device 10 (Y, C, M, K), and yellow is formed on the surface of the photoreceptor 1 (Y, C, M, K). A single color image of cyan, magenta, and black is formed. Then, along with the conveyance of the intermediate transfer belt 6, the monochrome images on the respective photoreceptors 1 (Y, C, M, K) are sequentially transferred to form a composite color image (color toner image) on the intermediate transfer belt 6. .

  When a start switch (not shown) is pressed, the paper feed roller 27 of the paper feed table 50 is driven to rotate, and the paper P is fed out from the paper feed cassette 26 provided in the paper feed table 50, and is separated by a separation roller (not shown). The sheets are separated one by one and enter the first paper feed path 29a. The paper P that has entered the first paper feed path 29 a is transported by the transport roller pair 30, guided to the second paper feed path 29 b in the printer 100 body, and abuts against the registration roller pair 28 and stops. Then, the registration roller pair 28 is rotationally driven in time with the composite color image on the intermediate transfer belt 6, and is moved to the secondary transfer nip between the intermediate transfer belt 6 and the secondary transfer roller 8 of the secondary transfer device 12. Paper P is fed. Then, the image is electrostatically transferred by the secondary transfer device 12 and a color image is recorded on the paper P.

  The sheet P after the image is transferred is transported from the secondary transfer device 12 and sent to the fixing device 13 through the transport guide member. Then, heat and pressure are applied at the fixing nip where the fixing roller 13 a and the pressure roller 13 b of the fixing device 13 face each other to fix the transferred image, and then the paper is discharged by the discharge roller 39 and placed on the paper discharge tray 40. Stacked.

  On the other hand, after the image is transferred, the residual toner remaining on the surface of the intermediate transfer belt 6 is removed by the intermediate transfer belt cleaning device 7, and the image forming devices 10 (Y, C, M, K) repeat the process. Prepare for image formation.

Next, a first density adjustment pattern image for forming a plurality of pattern images having different image densities with a constant image area, and density adjustment based on the pattern images will be described.
FIG. 3 is a block diagram illustrating an example of a main configuration of a control system that performs density adjustment in the printer 100. As shown in FIG. 3, the printer 100 includes a control unit 500 that performs drive control of each drive unit.

  The control unit 500 of the printer 100 controls the voltage applied to the primary transfer roller 17 by a primary transfer power source (not shown) so as to perform constant current control. Further, the charging potential by the charging device 2 in the image forming device 10 and the developing bias potential of the developing device 3 are controlled so that the image forming condition determined based on the detection result of the optical sensor 151 at the time of density adjustment is obtained. The intensity of the laser beam L of the apparatus 5 is controlled.

FIG. 4 is an explanatory diagram of a first density adjustment pattern image formed on the intermediate transfer belt 6.
The control unit 500 of the printer 100 executes image density control for optimizing the image density of each color when the power is turned on or whenever a predetermined number of images are formed.

  In the image density control, first, gradation pattern images (Sy, Sc, Sm, Sk) of each color as shown in FIG. 4 are applied to the optical sensors 151 (Y, C, M, K) on the intermediate transfer belt 6. Automatically formed at the opposite position. The gradation pattern image of each color is composed of ten square toner patches of 2 [cm] × 2 [cm] having different image densities. When the gradation pattern images (Sy, Sc, Sm, Sk) of the respective colors are created, the charging potential of the photoreceptor 1 (Y, C, M, K) is uniform in the image forming process during normal image formation. Unlike the photosensitive member charging potential, the value is gradually increased. Then, while forming a plurality of patch electrostatic latent images for forming a gradation pattern image by scanning with the laser beam L on each of the photoreceptors 1 (Y, C, M, K), the Y, C, Development is performed by the developing device 3 (Y, C, M, K) for M and K. During this development, the value of the developing bias applied to the Y, C, M, and K developing rollers is gradually increased.

By such development, gradation pattern images (Sy, Sc, Sm, Sk) of Y, C, M, K are formed on the four photoreceptors 1 (Y, C, M, K), respectively. . These gradation pattern images (Sy, Sc, Sm, Sk) are primarily transferred so as to be arranged at predetermined intervals in the main scanning direction of the intermediate transfer belt 6 as shown in FIG. At this time, the toner adhesion amount of the toner patch in the gradation pattern image of each color is about 0.1 [mg / cm ² ] at the minimum and 0.55 [mg / cm ² ] at the maximum, and the Q / When the distribution of d (average charge amount, “d” is the average particle diameter) distribution is measured, it is almost aligned with the normal charge polarity.

Each toner patch of each gradation pattern image (Sy, Sc, Sm, Sk) formed on the intermediate transfer belt 6 passes through a position facing the optical sensor 151 as the intermediate transfer belt 6 moves endlessly. At this time, the optical sensor 151 irradiates each gradation pattern image with light for detecting the toner adhesion amount, and receives an amount of reflected light corresponding to the toner adhesion amount per unit area with respect to the toner patch of each gradation pattern image. To do.
Next, from the output voltage of the optical sensor 151 when each color toner patch is detected and the adhesion amount conversion algorithm, the adhesion amount in each toner patch of the gradation pattern image of each color is calculated, and based on the calculated adhesion amount. Adjust the image conditions. Specifically, a function (y = ax + b) indicating a straight line graph is calculated by regression analysis based on the result of detecting the toner adhesion amount on the toner patch and the development potential when each toner patch is imaged. Then, an appropriate development potential is calculated by substituting the target value of image density into this function, the development potentials for Y, C, M, and K are specified, and the development bias value corresponding to each development potential is specified. To do.

The memory of the control unit 500 stores an image forming condition data table in which several tens of developing bias values and appropriate photosensitive member charging potentials corresponding to the developing bias values are associated in advance. For each image forming device 10 (Y, C, M, K), a developing bias value closest to the specified developing bias value is selected from the image forming condition table, and a photosensitive member charging potential associated therewith is selected. Identify. By performing image formation under the image formation conditions using the development bias value selected here and the specified photoconductor charging potential, the image density close to the target value even if the installation environment changes or deteriorates over time. Thus, stable image formation can be performed.
In the printer 100, the optical sensor 151 is a toner of gradation pattern images (Sy, Sc, Sm, Sk) of each color, which is a density adjustment pattern image formed on the photoreceptor 1 and transferred onto the intermediate transfer belt 6. This is toner adhesion amount detection means for detecting the adhesion amount. The control unit 500 is a control unit that controls image forming conditions of the image forming apparatus 10 that is an image forming unit based on the detection result of the optical sensor 151.

In the printer 100, the primary transfer bias applied to the primary transfer roller 17 is applied by constant current control. In the case of constant current control, the transfer electric field formed in the primary transfer nip is hardly affected by the resistance fluctuation of the transfer member such as the intermediate transfer belt 6 or the primary transfer roller 17, and stable transfer performance can be realized. It is possible to realize a transfer property having high robustness against resistance fluctuations such as the above.
However, in the configuration in which the primary transfer bias is controlled at a constant current, in order to maintain the constant current with respect to the fluctuation of the surface potential of the latent image carrier such as the photoconductor 1, the primary transfer is performed by the fluctuation of the surface potential of the latent image carrier. The primary transfer voltage applied to the primary transfer member such as the roller 17 varies.

  The fluctuation of the surface potential of the latent image carrier is caused by a decrease in the chargeability of the toner due to the deterioration of the developer due to use over time or a change in the chargeability of the toner due to a change in the use environment. Specifically, when the chargeability of the toner in the developing device that develops the latent image on the latent image carrier is low, the amount of toner attached to the latent image carrier relative to the development potential increases. The image forming conditions are controlled to be low. The storage unit of the image forming apparatus stores a plurality of combinations of developing biases and charging potentials corresponding to the developing potential. When the developing potential value is lowered, both the developing bias and the charging potential are low. It is controlled to become. As the charging potential is lowered, the surface potential of the latent image carrier is lowered. On the other hand, when the chargeability of the toner in the developing device is high, the surface potential of the latent image carrier is high.

  In the configuration of constant current control, when the surface potential of the latent image carrier becomes low, the primary transfer voltage applied to the primary transfer member is low in order to maintain a constant current between the latent image carrier and the primary transfer member. Become. On the other hand, as the surface potential of the latent image carrier increases, the primary transfer voltage also increases.

The printer 100 is a tandem intermediate transfer type image forming apparatus. When full-color printing is performed, toner images are sequentially primary-transferred from the photoreceptors 1 to the intermediate transfer belt 6.
In a tandem type image forming apparatus such as the printer 100, a toner image (hereinafter referred to as an “upstream toner image”) formed by an upstream image forming apparatus and transferred to an intermediate transfer member is formed on the downstream side. It passes through a primary transfer nip (hereinafter referred to as a “downstream transfer nip”) of the image apparatus. When the upstream toner image passes through the downstream transfer nip, reverse transfer occurs in which part of the toner forming the upstream toner image moves to the latent image carrier that contacts the intermediate transfer belt at the downstream transfer nip. . This reverse transfer tends to increase as the primary transfer voltage at the downstream transfer nip increases. When the downstream primary transfer voltage fluctuates, the ratio of the amount of toner transferred reversely to the amount of toner entering the downstream transfer nip (hereinafter, Also called “reverse transfer rate”).
Since the primary transfer voltage at the downstream transfer nip is high and the upstream toner image passes through the downstream transfer nip, reversely charged (plus charge) toner increases due to discharge, so the primary transfer voltage at the downstream transfer nip is The higher the reverse transcription rate, the worse.

Further, as a result of extensive studies by the present inventors, it has been found that the reverse transfer rate in the downstream transfer nip also varies depending on the charge amount (Q / M) of the toner forming the upstream toner image. . Specifically, it has been found that the reverse transfer rate decreases as the toner charge amount of the upstream toner image increases, and the reverse transfer rate increases as the toner charge amount of the upstream toner image decreases.
This is considered to be due to the following reasons.

That is, charge injection by discharge at the downstream transfer nip shifts the charged state of the toner forming the upstream toner image to a polarity opposite to the normal charge polarity of the toner. At this time, the weakly charged toner or the reversely charged toner moves to the latent image carrier at the downstream transfer nip, and reverse transfer is performed. Here, the larger the charge amount of the toner in the upstream toner image, the lower the ratio of the toner that becomes weakly charged toner or reversely charged toner even if the downstream transfer nip is subjected to charge injection by discharge. It will be smaller. Conversely, it is considered that the reverse transfer rate increases as the charge amount of the toner in the upstream toner image decreases, because the proportion of toner that becomes weakly charged toner or reversely charged toner by charge injection increases.
Since the reverse transcription rate fluctuates due to the influence of various factors, it is difficult to predict the amount of fluctuation in the reverse transcription rate by detecting the fluctuation of a part of the elements that affect the reverse transcription.

In the following, reverse transfer of magenta toner at the black transfer nip for primarily transferring the toner image on the black photoreceptor 1K of the black image forming device 10K to the intermediate transfer belt 6 will be described.
Note that the black image forming device 10K is arranged at a position on the most downstream side of the four image forming devices 10 (Y, C, M, K). For this reason, reverse transfer can be considered for each of the three color (yellow, cyan, magenta) toner images other than black in the black transfer nip. However, here, the reverse transfer of magenta toner formed by the magenta image forming apparatus 10M, which is the image forming apparatus 10 upstream one from the black image forming apparatus 10K, will be described.

  FIG. 5 shows a magenta transfer nip 60M to which the toner image on the magenta photoreceptor 1M is transferred to the intermediate transfer belt 6, and a black transfer nip 60K to which the toner image on the black photoreceptor 1K is transferred. FIG.

As shown in FIG. 5, the magenta toner image (Sm) on the magenta photoreceptor 1M is transferred to the intermediate transfer belt 6 at the magenta transfer nip 60M, conveyed by the surface movement of the intermediate transfer belt 6, and transferred to black. Passes through the nip 60K. Then, when passing through the black transfer nip 60K, a part of the magenta toner image is reversely transferred to the black photoreceptor 1K by a primary transfer bias or the like in the black transfer nip 60K.
This reverse transfer tends to increase as the primary transfer voltage in the downstream transfer nip such as the black transfer nip 60K increases. For this reason, when the primary transfer voltage in the downstream transfer nip varies, the ratio of the reversely transferred toner amount to the toner amount entering the downstream transfer nip (hereinafter referred to as “reverse transfer rate”) also varies.

The reverse transfer rate fluctuates when the primary transfer voltage fluctuates, and this primary transfer voltage fluctuates when the surface potential of the photoreceptor 1 facing the primary transfer roller 17 at the primary transfer nip fluctuates in constant current control. Here, the surface potential of the photosensitive member 1 is an average potential of the exposed portion on the surface of the photosensitive member 1 exposed after being uniformly charged and the other portion.
Further, the surface potential of the photoconductor 1 fluctuates when the charge amount of the toner in the developing device 3 fluctuates due to changes over time or environmental changes.

First, fluctuations in the surface potential of the photoreceptor 1 caused by fluctuations in the toner charge amount will be described.
FIG. 6 is an explanatory diagram showing fluctuations in the primary transfer voltage due to fluctuations in the surface potential of the photoreceptor 1 in constant current control.
When the charge amount of the toner is large, the absolute value of the surface potential of the photoconductor 1 becomes high, and when the charge amount of the toner is small, the absolute value of the surface potential of the photoconductor 1 becomes low. This is due to the following reason.

  The toner in the developing device 3 is deteriorated in charging property due to deterioration or environmental change with time, and the charge amount is reduced. When the charge amount of the toner is reduced, more toner can be developed even with the same developing potential (potential difference between the developing bias Vb and the exposed portion potential VL), and the developing ability is increased. For this reason, the development potential required to obtain the same image density is a low value.

In the process control, with the background potential (potential difference between the developing bias Vb and the charging potential Vd) kept constant, a density pattern is read by creating a gradation pattern image with the developing bias Vb and the charging potential Vd leveled. Adjust the density with.
When the charge amount of the toner is small and the developing ability is high, the development bias Vb and the charging potential Vd corresponding to the low development potential are set by the process control. At this time, the charging potential Vd is a value on the negative polarity (regular charging polarity of toner) side, as shown by Vd2 in FIG. 6B, and its absolute value is set to a low value.
On the other hand, when the charge amount of the toner is large and the developing ability is low, the development bias Vb and the charging potential Vd corresponding to the high value development potential are set by the control of the process control. At this time, the charging potential Vd has a value on the negative polarity (normal charging polarity of toner) side as Vd1 in FIG. 6A, and its absolute value is set to a high value.

  In response to such a change in the surface potential of the photoreceptor 1, in the constant current control, as shown in FIG. 6, a certain potential difference ΔV is generated, so that the primary transfer voltages (V 11, V 12) are the photoreceptor 1. The higher the absolute value of the surface potential, the lower the value. Therefore, in the example shown in FIG. 6, the primary when the absolute value of the charging potential Vd is lower (Vd2) than the primary transfer voltage (V11) when the absolute value of the charging potential Vd is high (Vd1). The transfer voltage (V12) has a higher value.

Next, the variation in the reverse transfer rate due to the variation in the primary transfer voltage will be described.
FIG. 7 is a graph showing the relationship between the change in the primary transfer voltage of the downstream transfer nip and the change in the reverse transfer rate of the upstream toner image. The graph (1) in FIG. 7 is a graph when the charge amount of the upstream toner image is large, and the graph (2) is a graph when the charge amount of the upstream toner image is small.

The upstream toner image transferred at the upstream transfer nip such as the magenta transfer nip 60M is negatively charged, but reverse transfer is performed according to the primary transfer voltage applied at the downstream transfer nip such as the black transfer nip. The rate is different.
In the area A in the graph shown in FIG. 7, since the primary transfer voltage at the downstream transfer nip is not so high, the reverse transfer rate of the upstream toner image hardly changes even when the primary transfer voltage is changed. On the other hand, in the region B in the graph shown in FIG. 7, the reverse transfer rate tends to increase as the primary transfer voltage increases. This is because as the primary transfer voltage increases, the amount of charge injected by charge injection by discharge at the downstream transfer nip increases, and the amount of toner that is reversely charged (plus charged) increases. The reverse transfer rate increases.

Even when the primary transfer voltage in the region A is 0 [V], the reverse transfer rate may not become 0 [%] due to the shear force generated by the linear velocity difference between the photosensitive member 1 and the intermediate transfer belt 6. .
The amount of increase in the reverse transfer rate with respect to the increase in the primary transfer voltage in the region B varies depending on the resistance value of the intermediate transfer belt 6, the resistance value of the primary transfer roller 17, and the positional relationship between the primary transfer roller 17 and the photoreceptor 1. .
The printer 100 according to this embodiment has the following settings. When the volume resistivity of the intermediate transfer belt 6 is 1.0 × 10 ^10.5 [Ω · cm] and the volume resistivity of the primary transfer roller 17 is 1.0 × 10 ⁷ [Ω · cm], the primary transfer roller The center position of 17 was set to be 3 [mm] downstream in the horizontal direction with respect to the center position of the photoreceptor 1. In such a printer 100, with respect to the primary transfer voltage in a range where the primary transfer voltage is in the region B and the graph is a linear approximation in a state where the toner charge amount of the upstream toner image is small (graph (2) in the figure). The increase rate of the reverse transfer rate was 0.01 [% / V]. At this time, the charge amount of the toner (toner after primary transfer) on the intermediate transfer belt 6 on the upstream side of the downstream transfer nip of the upstream toner image was −50 [μC / g].

  The higher the primary transfer voltage at the downstream transfer nip, the higher the reverse transfer rate, but when the absolute value of the toner charge amount of the upstream toner image before passing through the downstream transfer nip is large, the graph of (1) in FIG. As shown, the slope of the reverse transfer rate with respect to the primary transfer voltage becomes gentle. Here, the absolute value of the toner charge amount of the upstream toner image is a value on the normal charge polarity side of the toner, and in this embodiment, is a negative polarity charge amount. As the toner charge amount of the upstream toner image is larger, reverse charge (plus charge) is less likely to occur even if a discharge occurs in the downstream transfer nip, and the slope of the reverse transfer rate with respect to the primary transfer voltage becomes gentler.

The toner charge amount of the upstream toner image varies depending on the fluctuation of the toner charge amount in the developing device 3 of the upstream image forming device such as the magenta image forming device 10M.
For example, when the toner charge amount in the developing device 3 included in the upstream image forming device increases in a low temperature and low humidity environment, the toner charge amount of the upstream toner image formed by the toner increases and reverse rotation at the downstream transfer nip occurs. The copy rate is unlikely to be high.

Next, fluctuations in the reverse transfer rate caused by fluctuations in the film thickness of the surface layer of the photoreceptor 1 will be described.
FIG. 8 is an explanatory diagram showing fluctuations in the primary transfer voltage in constant current control due to fluctuations in the layer thickness of the surface layer of the photoreceptor 1 in constant current control.

When the surface layer of the photoreceptor 1 is thick, the electrostatic capacity of the photoreceptor 1 is lowered. This is due to the following reason. The capacitance can be expressed by the following equation (1).
C = ε · S / d (1)
In the above equation (1), “C” is the capacitance, “ε” is the dielectric constant, “S” is the surface area of the photoreceptor 1, and “d” is the film thickness of the surface layer of the photoreceptor 1.

The relationship between potential and charge in the capacitor model is as shown in the following equation (2).
Q = C ・ V (2)
In the above equation (2), “Q” is a charge amount, and “V” is a potential.
Further, the relational expression between the charge and the current is as shown in the following expression (3).
Q = I · t (3)
In the above equation (3), “I” is current and “t” is time.

From the relationship of the above formulas (1) to (3), when the current value is “I”, the relationship of the following formula (4) is established.
I = ε · S · V / (d · t) (4)

From the above equation (4), when the surface layer of the photoconductor 1 is thick, the value of “d” increases, so the voltage “V” is increased to allow the same current “I” to flow in constant current control. There is a need. For this reason, if the surface layer of the photoreceptor 1 is thick even at the same transfer current value, the transfer voltage (V13) rises as shown in FIG. 8B, and even if the target current value for constant current control is low, The transfer rate will rise.
On the other hand, when the film thickness of the surface layer of the photoconductor 1 becomes thin due to aging, the transfer voltage (V11) decreases as shown in FIG. The potential difference between the image carrier and the image carrier is not increased, and the transfer rate does not rise. Therefore, when the film thickness of the surface layer of the photosensitive member 1 is reduced, the optimum current value (target current value for constant current control) for obtaining a desired transfer rate may be different from that when the film thickness is thick. To do.

As described above, the way in which the transfer rate rises differs depending on the film thickness of the surface layer of the photoreceptor 1. The reverse transfer in the downstream transfer nip depends on the film thickness of the surface layer of the photoreceptor 1 that forms the downstream transfer nip. The rates can also be different.
That is, in the initial use state where the surface layer of the photoconductor 1 is thick, the primary transfer voltage when constant current control is performed with a certain target current value increases, and therefore the reverse transfer rate tends to increase. Conceivable. On the other hand, it is considered that the reverse transfer rate tends to be low in the state after use over time when the film thickness of the surface layer of the photoconductor 1 is reduced by use and thinned.

  Even if the thickness of the surface layer of the photoreceptor 1 forming the downstream transfer nip is thick and the reverse transfer rate tends to be high, if the toner charge amount of the upstream toner image is large, the reverse transfer rate The rise is suppressed. Even if the surface layer of the photoreceptor 1 forming the downstream transfer nip is thin and the reverse transfer rate tends to be low, the reverse transfer rate can be reduced if the toner charge amount of the upstream toner image is small. Rises.

FIG. 1 is an enlarged explanatory view of four primary transfer nips 60 (Y, C, M, K) on the surface of the intermediate transfer belt 6 and detection positions by the optical sensor 151.
As shown in FIG. 1, a transfer nip downstream side optical sensor 152 (Y, C, M) for detecting the toner adhesion amount on the intermediate transfer belt 6 is provided between the primary transfer nips 60.
With such a configuration, the toner adhesion amount of the upstream toner image before and after passing through the primary transfer nip 60 (Y, C, M, K) located downstream of the other primary transfer nip 60 is detected, It is possible to detect a change in the toner adhesion amount before and after passing through the primary transfer nip 60.

Specifically, the toner adhesion amount of the yellow toner image (yellow gradation pattern image Sy) that is the upstream toner image before passing through the cyan transfer nip 60C can be detected by the yellow transfer nip downstream optical sensor 152Y. The toner adhesion amount of the yellow toner image after passing through the cyan transfer nip 60C can be detected by the cyan transfer nip downstream optical sensor 152C.
Similarly, the toner adhesion amount of the upstream toner image (Sy, Sc) before passing through the magenta transfer nip 60M can be detected by the cyan transfer nip downstream optical sensor 152C. The toner adhesion amount of the upstream toner image (Sy, Sc) after passing through the magenta transfer nip 60M can be detected by the magenta transfer nip downstream optical sensor 152M.

  Further, the toner adhesion amount of the upstream toner image (Sy, Sc, Sm) before passing through the black transfer nip 60K can be detected by the magenta transfer nip downstream optical sensor 152M. The toner adhesion amount of the upstream toner image (Sy, Sc, Sm) after passing through the black transfer nip 60K can be detected by the optical sensor 151.

Based on the detected value of the upstream toner adhesion amount before passing through the downstream transfer nip and the detected value of the upstream toner adhesion amount after passing through the downstream transfer nip, the control unit 500 performs the downstream transfer. The reverse transfer rate of the upstream toner image at the nip is calculated.
When the upstream toner adhesion amount before passing through the downstream transfer nip is T1 [g / cm ² ] and the upstream toner adhesion amount after passing through the downstream transfer nip is T2 [g / cm ² ]. The reverse transfer rate Ta [%] can be calculated by the following equation (5).
Ta = (T1-T2) / T1 × 100 (5)

  In the tandem intermediate transfer type printer 100, when a full-color image is formed, the primary transfer nip 60 (C, M, K) passes through the primary transfer nip 60 (Y) on the most upstream side to the secondary transfer nip. To do.

  In the conventional tandem intermediate transfer type image forming apparatus, the gradation pattern image is read only by the optical sensor 151 on the downstream side of the black transfer nip 60K. In this method, the toner image forming the upstream side toner image (tone pattern image) is finally transferred to the most downstream primary transfer nip 60 (K) without knowing how much reverse transfer is performed in which primary transfer nip 60. After passing, the toner adhesion amount of the upstream toner image is read.

  On the other hand, in the printer 100 of the present embodiment shown in FIG. 1, the transfer nip downstream optical sensor 152 (Y, C, M) is provided between the primary transfer nips 60, and the downstream side of the most downstream primary transfer nip 60. Includes an optical sensor 151. This makes it possible to detect how much the toner forming the upstream toner image (tone pattern image) has been reversely transferred at which primary transfer nip 60.

As shown in FIG. 1, the printer 100 is configured to read the image density (toner adhesion amount) between the primary transfer nips 60 (Y, C, M, K). In addition, on the back side of the intermediate transfer belt 6 at the sensor reading position where each transfer nip downstream optical sensor 152 (Y, C, M) reads the image density, a sensor facing member 153 (Y, C, M) are arranged.
In the configuration example shown in FIG. 1, the sensor facing member 153 (Y, C, M) slides on the belt as a non-rotating member having a rectangular parallelepiped shape. However, the roller facing member eliminates friction with the belt. The life of the belt can be extended.

FIG. 9 is an enlarged explanatory view of the four primary transfer nips 60 (Y, C, M, K) on the surface of the intermediate transfer belt 6 and the detection position by the optical sensor 151 when a monochrome image is formed.
As shown in FIG. 9, when creating a monochrome image, the primary transfer roller 17 (Y, C, M) used for primary transfer of the color image is separated from the intermediate transfer belt 6. Accordingly, as shown in FIG. 9, the color image photoreceptor 1 (Y, C, M) and the primary transfer roller 17 (Y, C, M) are not in contact with the intermediate transfer belt 6.
The non-contact prevents the rubbing against each other except when necessary, and the photoreceptor 1 (Y, C, M), the primary transfer roller 17 (Y, C, M), and the intermediate transfer belt 6. And a longer life.

  Further, as shown in FIGS. 1 and 9, the sensor facing member 153 (Y, C, M) is synchronized with the contact / separation operation of the primary transfer roller 17 (Y, C, M) located on the upstream side thereof. Thus, the intermediate transfer belt 6 is brought into contact with and separated from the intermediate transfer belt 6. Accordingly, when a monochrome image is formed, the sensor facing member 153 (Y, C, M) is in a state of being separated from the intermediate transfer belt 6 as shown in FIG. Thereby, it is possible to prevent the sensor facing members 153 (Y, C, M) and the intermediate transfer belt 6 from extending their lives by preventing them from rubbing each other except when necessary.

  Further, a shutter member (not shown) is provided between each of the optical sensor 151 and the transfer nip downstream side optical sensor 152 (Y, C, M) and the intermediate transfer belt 6. When each sensor (151, 152) does not read the image density (toner adhesion amount), the shutter member shields the sensor from the intermediate transfer belt 6 to prevent scattered toner from adhering to the sensor. .

It is desirable that the electrical resistance of the sensor facing member 153 (Y, C, M) be higher than the electrical resistance of the primary transfer roller 17. As a result, it is possible to prevent the charge generated when the primary transfer bias is applied from flowing into the sensor facing member 153 (Y, C, M) instead of the photoreceptor 1.
In particular, when the primary transfer roller 17 is a metal roller and the inner surface resistance of the intermediate transfer belt 6 is 1.0 × 10 ¹⁰ [Ω / □], the sensor facing member 153 (Y, C, M) is charged. Easy to flow. For this reason, it is desirable that the volume resistivity of the sensor facing member 153 (Y, C, M) is 1.0 × 10 ⁷ [Ω · cm] or more.

FIG. 10 is an explanatory diagram showing transfer current value control based on the calculation result of the reverse transfer rate of the downstream transfer nip.
First, the gradation pattern image of the upstream toner image in which the developing bias Vb and the charging potential Vd are leveled is transferred onto the intermediate transfer belt 6 during process control. Then, the image density is read by sensors (151 and 152) arranged on the upstream side and the downstream side of the downstream transfer nip, respectively.

When the upstream toner image is the magenta gradation pattern image Sm, the reverse transfer rate (Tam) at the black transfer nip 60K that is the downstream transfer nip can be calculated by the following equation (6).
Tam = (T1m−T2m) / T1m × 100 (6)
In the above equation (6), “T1m” is the toner adhesion amount of the magenta gradation pattern image Sm detected by the magenta transfer nip downstream optical sensor 152M. “T2m” is the toner adhesion amount of the magenta gradation pattern image Sm detected by the optical sensor 151.

At the same time, the development bias Vb and the charging potential Vd that attain the target toner adhesion amount at the secondary transfer position are determined based on the detection result of the optical sensor 151 in the process control described above.
Under the conditions of the development bias Vb and the charging potential Vd determined here, the reverse transfer rate at the downstream transfer nip such as the black transfer nip 60K is calculated based on the detection results of the sensors (151 and 152).

If the reverse transfer rate increases due to aging or environmental changes and the reverse transfer rate exceeds a preset threshold, the target current value for constant current control in the downstream transfer nip where the reverse transfer rate exceeds the threshold is lowered. Control to do.
By performing control to lower the target current value of constant current control, the primary transfer voltage in the downstream transfer nip becomes lower than before control as shown in FIG. 10, and the reverse transfer rate can be lowered. However, if the primary transfer voltage is lowered, the primary transfer rate from the photoreceptor 1 forming the downstream transfer nip to the intermediate transfer belt 6 may also be reduced.

  FIG. 11 is a graph showing an example of the relationship between the transfer rate and the change in the primary transfer voltage when the primary transfer voltage is changed by changing the target value of constant current control in the downstream transfer nip. The broken graph in FIG. 11 shows the fluctuation of the transfer rate of the primary transfer (hereinafter also referred to as “normal transfer rate”).

  As shown in FIG. 11, the normal transfer rate changes so as to draw a mountain parabola as the transfer voltage increases. For this reason, in the range where the transfer rate is high, such as the range between “α1” and “α2” in FIG. 11, the fluctuation range of the transfer rate with respect to the change in transfer voltage is small. In addition, when the transfer voltage becomes larger than the transfer voltage at which the transfer rate is maximized, the transfer rate is lowered. FIG. 11 is a graph in a case where the transfer voltage is constant except for the transfer voltage. The relationship of the transfer rate fluctuation to the transfer voltage fluctuation is not constant as shown in FIG. 11, and the toner transferred at the downstream transfer position is not constant. It fluctuates due to changes in the charge amount and usage environment.

On the other hand, as shown in FIG. 10, the reverse transfer rate is substantially constant in the range where the transfer voltage at the downstream transfer position is low, and when the transfer voltage becomes larger than a certain value, the reverse transfer rate is high with respect to the increase amount of the transfer voltage. The reverse transcription rate increases linearly with increasing amount.
FIG. 10 is a graph in a case where the transfer voltage is constant except for the transfer voltage, and the relationship of the reverse transfer rate variation to the transfer voltage variation is not constant as shown in FIG. 10, but the upstream side passing through the downstream transfer position. It fluctuates due to fluctuations in the charge amount of the toner forming the toner image and the usage environment. Therefore, even if the transfer voltage at the downstream transfer position does not fluctuate, the reverse transfer rate may deteriorate due to fluctuations in the charge amount of the toner forming the upstream toner image, the usage environment, and the like.

The transfer voltage of the downstream transfer unit when the reverse transfer rate is deteriorated due to the charge amount of the toner forming the upstream toner image or the change in the use environment is included in any of the following three ranges.
The first is a range where the fluctuation range of the transfer rate with respect to the fluctuation of the transfer voltage is small as between “α1” and “α2” in FIG.
The second is a range where the transfer voltage is higher than the first range (the range on the left side of “α2” in FIG. 11).
The third is a range where the transfer voltage value is lower than the first range (a range on the left side of “α1” in FIG. 11).

When the value of the transfer voltage of the downstream transfer means when the reverse transfer rate is deteriorated is included in the first range, the effect of reducing the reverse transfer rate by reducing the transfer voltage is large, and the transfer voltage is lowered. The influence of fluctuation of the normal transfer rate due to this is reduced. Therefore, the reverse transfer rate is lowered while maintaining the normal transfer rate at the downstream transfer position by performing control to lower the target current value of the constant current control so as to lower the transfer voltage at the downstream transfer position. be able to.
Generally, the target current value in the constant current control is set so as to ensure a potential difference with a high normal transfer rate. For this reason, there is a high possibility that the transfer voltage of the downstream transfer means when the reverse transfer rate is deteriorated is included in the first range.

  When the transfer voltage when the reverse transfer rate deteriorates is included in the second range, the transfer voltage is higher than the transfer voltage at which the normal transfer rate is maximized. The rate will also be high. Therefore, the reverse transfer rate is lowered while improving the normal transfer rate at the downstream transfer position by performing control to lower the target current value of the constant current control so as to lower the transfer voltage at the downstream transfer position. be able to.

  If the transfer voltage when the reverse transfer rate deteriorates is included in the third range, the transfer rate at the downstream transfer position is lowered by lowering the target current value, and a toner image with a desired density is obtained. It may not be possible. In this case, it is possible to compensate by setting the image forming conditions so as to increase the toner adhesion amount of the toner image formed on the downstream photoreceptor 1 by adjusting the density.

  In the printer 100, when the control for reducing the target current value of the constant current control is performed in order to reduce the reverse transfer rate, the development bias Vb and the charging potential Vd, which are the target toner adhesion amount, are once again at the end of the process control. A pattern image is formed on the intermediate transfer belt 6 under conditions.

Then, the optical sensor 151 determines whether the pattern image transferred at the downstream transfer nip that has been controlled to reduce the target current value of the constant current control has the target amount of toner adhesion at the secondary transfer position. Check the amount. If the target toner adhesion amount is not reached, the process control is performed with the target current value after the control to lower the target current value of constant current control, and the development bias Vb and the charge that becomes the target toner adhesion amount are charged. A combination with the potential Vd is determined.
As a result, the toner that forms the upstream toner image can be prevented from being excessively consumed due to an increase in the reverse transfer rate at the downstream transfer nip.

  As described with reference to FIG. 8, when the surface layer of the photoreceptor 1 is scraped and the film thickness becomes thin with time, the rise of the primary transfer rate with respect to the target current value in the constant current control is delayed. In some cases, the target current value cannot be set low. In this case, excessive toner consumption can be prevented by urging the replacement of the photosensitive unit for attaching / detaching the photosensitive member 1 to / from the printer 100.

  As described above, the printer 100 of the present embodiment is an image forming apparatus in which the primary transfer nip 60 is controlled with constant current. Further, by increasing the reverse transfer rate in the downstream transfer nip such as the black transfer nip 60K, waste of toner in the upstream image forming apparatus such as the magenta image forming apparatus 10M is suppressed. Specifically, image density sensors (151 and 152) are arranged upstream and downstream of the downstream transfer nip. In the process control, the reverse transfer rate in the downstream transfer nip of the upstream toner image formed by the upstream image forming device is detected, and when the reverse transfer rate increases and exceeds the threshold, the downstream side Control is performed to lower the target current value of constant current control at the transfer nip.

  When the toner charge amount decreases with the aging of the developer in the developing device 3 that forms the toner image transferred at the downstream transfer nip, the surface potential (non-exposed portion) of the photoreceptor 1 is controlled by process control. ) Becomes smaller. As a result, the primary transfer voltage increases and the reverse transfer rate with respect to an arbitrary target current value deteriorates. In such a case, the printer 100 executes a control for decreasing the target current value of the constant current control in the downstream transfer nip every time it detects that the reverse transfer rate exceeds the threshold during process control. Will be.

In order to suppress an increase in the reverse transfer rate, the downstream transfer nip is arranged so that reverse transfer is unlikely to occur even when the toner charge amount decreases and the absolute value of the surface potential of the photoreceptor 1 decreases. A configuration is possible in which the target current value in the constant current control is set to a low value in advance. However, with this configuration, the transfer rate in the downstream transfer nip cannot be ensured, and appropriate setting is difficult.
In addition, the transfer rate with respect to the transfer voltage tends to increase as the transfer voltage increases. However, the transfer rate does not increase even if the transfer voltage is increased when the transfer voltage reaches a certain value. If the transfer voltage is further increased, the transfer rate may decrease. If the relationship between the transfer voltage and the transfer rate at the downstream transfer nip is higher than the transfer voltage reaches a certain value, the transfer rate will decrease even if the target current value is decreased to reduce the reverse transfer rate. In some cases, it is possible to achieve both reduction in the reverse transfer rate and maintenance of the transfer rate.

On the other hand, when the film of the surface layer of the photoconductor 1 is cut and thinned due to deterioration of the photoconductor 1 with time, the electric resistance in the primary transfer is reduced by the film thickness that has not been cut away. In the constant current control, the primary transfer voltage is lowered. For this reason, when the surface layer of the photoreceptor 1 forming the downstream transfer nip is thinned, a reverse transfer rate with respect to an arbitrary target current value in the constant current control hardly occurs. However, when the transfer voltage is lowered, the transfer rate of the downstream toner image to be transferred at the downstream transfer nip may be reduced. For this reason, with respect to the phenomenon in which the surface layer of the photoreceptor 1 forming the downstream transfer nip is thinned, control for increasing the target current value of constant current control in the downstream transfer nip is executed. .
When the control to increase the target current value is performed, the reverse transfer rate tends to deteriorate. Therefore, if the transfer rate of the downstream toner image can be maintained and the deterioration of the reverse transfer rate cannot be suppressed, the photoconductor Encourage unit replacement.

By performing the above-described control, the printer 100 does not consume extra toner for fluctuations in the film thickness of the surface layer of the photosensitive member 1 over time and fluctuations in the charging potential of the photosensitive member 1 due to developer deterioration. Stable image quality can be provided.
The printer 100 detects the toner adhesion amount before and after the gradation pattern image (Sy, Sc, Sm), which is a density adjustment pattern image, passes through the downstream transfer nip (60C, 60M, 60K), and based on the difference. Thus, the reverse transcription rate is calculated. The toner image used for calculating the reverse transfer rate is not limited to the gradation pattern image. However, by using the gradation pattern image for calculating the reverse transfer rate, it is not necessary to separately form a toner image for calculating the reverse transfer rate.

  In the printer 100 of the above-described embodiment, all of the four primary transfer nips 60 are configured to perform constant current control. This is not a limitation. The present invention is applicable as long as at least one of the downstream transfer nips having the other primary transfer nip 60 on the upstream side performs constant current control.

What has been described above is merely an example, and the present invention has a specific effect for each of the following modes.
(Aspect A)
Toner image forming means such as a plurality of image forming devices 10 that respectively form toner images on the surface of a plurality of latent image carriers such as photoreceptors 1 that move on the surface based on image information, and toner on the latent image carrier A plurality of primary transfer rollers 17 that respectively transfer the image to an intermediate transfer member such as an intermediate transfer belt 6 that moves on the surface, transfer means such as a primary transfer power source (not shown), and a toner image on the intermediate transfer member are recorded on a sheet P or the like. In an image forming apparatus such as a printer 100 having a recording medium transfer unit such as a secondary transfer device 12 that transfers to a medium, of the plurality of transfer units, the upstream side in the direction of surface movement of the intermediate transfer member relative to the transfer unit The downstream transfer means, such as the black primary transfer roller 17K, having at least one other transfer means such as the magenta primary transfer roller 17M, controls the transfer voltage by constant current control. The upstream toner image such as the magenta tone pattern image Sm transferred to the intermediate transfer member by the other transfer means upstream of the downstream transfer position such as the black transfer nip 60K where the transfer is performed is transferred to the downstream side. A magenta transfer nip downstream optical sensor 152M, an optical sensor 151, and a control unit 500 are provided with toner adhesion amount fluctuation detection means for detecting fluctuations in the toner adhesion amount of the upstream toner image before and after passing through the position. The means changes the value of the target current in the constant current control based on the detection result of the toner adhesion amount fluctuation detection means.
According to this, as described in the above embodiment, the reverse transfer rate at the downstream transfer position is determined by detecting the change in the toner adhesion amount of the upstream toner image before and after passing through the downstream transfer position. Can be calculated. When the reverse transfer rate exceeds a preset threshold value, the transfer voltage at the downstream transfer position is lowered by changing the target current value in the constant current control of the downstream transfer unit to decrease. The reverse transcription rate can be lowered. Accordingly, it is possible to suppress excessive consumption of toner for forming the upstream toner image due to an increase in the reverse transfer rate at the downstream transfer position.
In the main body mode, as described above, control is performed to lower the target current value of the downstream transfer unit having at least one other transfer unit upstream of the transfer unit in the surface movement direction of the intermediate transfer member. Yes. Since the downstream transfer means uses constant current control, if the target current value is lowered, the potential difference between the potential of the latent image carrier and the transfer voltage at the downstream transfer position becomes smaller. There is a concern that the normal transfer rate from the latent image carrier to the intermediate transfer member may decrease.
As shown in FIG. 11, the normal transfer rate changes so as to draw a mountain parabola with respect to an increase in transfer voltage. For this reason, in the range where the transfer rate is high and in the vicinity of the peak, as in the range between “α1” and “α2” in FIG. 11, the fluctuation range of the transfer rate with respect to the transfer voltage is small. That is, the amount of change in the vertical axis relative to the change in the horizontal axis in FIG. In addition, when the transfer voltage becomes larger than the transfer voltage at which the transfer rate is maximized, the transfer rate is lowered.
FIG. 11 is a graph in a case where the transfer voltage is constant except for the transfer voltage. The relationship of the transfer rate fluctuation to the transfer voltage fluctuation is not constant as shown in FIG. 11, and the toner transferred at the downstream transfer position is not constant. It fluctuates due to changes in the charge amount and usage environment.
On the other hand, as shown in FIG. 10, the reverse transfer rate is substantially constant in the range where the transfer voltage at the downstream transfer position is low, and when the transfer voltage becomes larger than a certain value, the reverse transfer rate is high with respect to the increase amount of the transfer voltage. The reverse transcription rate increases linearly with increasing amount.
FIG. 10 is a graph in a case where the transfer voltage is constant except for the transfer voltage, and the relationship of the reverse transfer rate variation to the transfer voltage variation is not constant as shown in FIG. 10, but the upstream side passing through the downstream transfer position. It fluctuates due to fluctuations in the charge amount of the toner forming the toner image and the usage environment. Therefore, even if the transfer voltage at the downstream transfer position does not fluctuate, the reverse transfer rate may deteriorate due to fluctuations in the charge amount of the toner forming the upstream toner image, the usage environment, and the like.
The transfer voltage of the downstream transfer unit when the reverse transfer rate is deteriorated due to the charge amount of the toner forming the upstream toner image or the change in the use environment is included in any of the following three ranges.
The first is a range where the fluctuation range of the transfer rate with respect to the fluctuation of the transfer voltage is small as between “α1” and “α2” in FIG.
The second is a range where the transfer voltage is higher than the first range (the range on the left side of “α2” in FIG. 11).
The third is a range where the transfer voltage value is lower than the first range (a range on the left side of “α1” in FIG. 11).
When the value of the transfer voltage of the downstream transfer means when the reverse transfer rate is deteriorated is included in the first range, the effect of reducing the reverse transfer rate by reducing the transfer voltage is large, and the transfer voltage is lowered. The influence of fluctuation of the normal transfer rate due to this is reduced. Therefore, the reverse transfer rate is lowered while maintaining the normal transfer rate at the downstream transfer position by performing control to lower the target current value of the constant current control so as to lower the transfer voltage at the downstream transfer position. be able to.
Generally, the target current value in the constant current control is set so as to ensure a potential difference with a high normal transfer rate. For this reason, there is a high possibility that the transfer voltage of the downstream transfer means when the reverse transfer rate is deteriorated is included in the first range.
When the transfer voltage when the reverse transfer rate deteriorates is included in the second range, the transfer voltage is higher than the transfer voltage at which the normal transfer rate is maximized. The rate will also be high. Therefore, the reverse transfer rate is lowered while improving the normal transfer rate at the downstream transfer position by performing control to lower the target current value of the constant current control so as to lower the transfer voltage at the downstream transfer position. be able to.
When the transfer voltage when the reverse transfer rate is deteriorated is included in the third range, the normal transfer rate at the downstream transfer position is lowered by lowering the target current value, and a toner image having a desired density is obtained. May not be obtained. In this case, it is possible to compensate by setting the image forming condition so as to increase the toner adhesion amount of the toner image formed on the latent image carrier by adjusting the density.
(Aspect B)
In the aspect A, the surface of the intermediate transfer body such as the intermediate transfer belt 6 is moved in the downstream direction with respect to the transfer position such as the primary transfer nip 60 of each of the plurality of primary transfer rollers 17 and transfer means such as a primary transfer power source (not shown) In addition, a toner adhesion amount detection sensor such as an optical sensor 152 (Y, C, M) or an optical sensor 151 on the downstream side of the transfer nip for detecting the toner adhesion amount of the toner image on the surface of the intermediate transfer member is provided, and the control unit 500 The toner adhering amount fluctuation detecting means includes a magenta gradation detected by a toner adhering amount detecting sensor such as a magenta transfer nip downstream optical sensor 152M disposed upstream of a downstream transfer position such as the black transfer nip 60K. By the toner adhesion amount of the upstream toner image such as the pattern image Sm and the toner adhesion amount detection sensor such as the optical sensor 151 disposed downstream of the downstream transfer position. A toner adhesion amount of the detected upstream side toner image Te, calculates the difference.
According to this, as described in the above embodiment, a configuration for calculating the reverse transfer rate at the downstream transfer position can be realized.
(Aspect C)
In mode B, a shutter member (not shown) that shields between the toner adhesion amount detection sensor such as the optical sensor 152 (Y, C, M) or the optical sensor 151 on the downstream side of the transfer nip and the intermediate transfer body such as the intermediate transfer belt 6. A sensor shutter member such as
According to this, as described in the above embodiment, the provision of the sensor shutter member can prevent the toner on the intermediate transfer member from adhering to the toner adhesion amount detection sensor and preventing the toner adhesion amount from being read. .
(Aspect D)
In either aspect B or C, the intermediate transfer member is an intermediate transfer belt such as a belt-shaped intermediate transfer belt 6, and toner adhesion amount detection such as a transfer nip downstream optical sensor 152 (Y, C, M) is detected. A detection position counter member such as a sensor facing member 153 (Y, C, M) that contacts the back surface of the intermediate transfer belt at a position where the sensor faces is provided.
According to this, as described in the above embodiment, the reflected light of the laser on the front surface of the intermediate transfer belt at the position where the toner adhesion amount detection sensor faces can be stabilized, and the toner adhesion amount detection sensor The detection accuracy can be improved.
(Aspect E)
In aspect D, the plurality of transfer units each include a transfer member such as a primary transfer roller 17 that faces a latent image carrier such as the photoreceptor 1 with the intermediate transfer belt such as the intermediate transfer belt 6 interposed therebetween, The detection position counter member such as the sensor facing member 153 is displaced in synchronism with the contact / separation operation of the transfer member.
According to this, as described in the above embodiment, it is possible to prevent each other from rubbing except when necessary, and to extend the life of the detection position counter member and the intermediate transfer belt.
(Aspect F)
In any of the aspects D or E, the plurality of transfer units include a transfer member such as a primary transfer roller 17 that faces a latent image carrier such as the photosensitive member 1 across the intermediate transfer belt such as the intermediate transfer belt 6. The electrical resistance of the detection position opposing member such as the sensor facing member 153 provided is higher than the electrical resistance of the transfer member.
According to this, as described in the above embodiment, by increasing the electric resistance of the detection position counter member, the charge applied from the transfer member is prevented from flowing into the detection position counter member, and flows into the latent image carrier. It is possible to prevent a decrease in image density due to insufficient charge.
(Aspect G)
In any one of the aspects D to F, the detection position opposing member such as the sensor facing member 153 has a roller shape.
According to this, as described in the above embodiment, the detection position counter member is a roller-shaped rotating body, thereby suppressing the friction between the detection position counter member and the intermediate transfer belt such as the intermediate transfer belt 6. Thus, the life of the intermediate transfer belt can be extended as compared with a configuration using a sliding member.

1Y yellow photoconductor 1K black photoconductor 1M magenta photoconductor 1 photoconductor 2 charging device 3 developing device 4 cleaning device 5 exposure device 6 intermediate transfer belt 7 intermediate transfer belt cleaning device 8 secondary transfer roller 9 pressure member 10 Image forming device 10Y Image forming device for yellow 10K Image forming device for black 10M Image forming device for magenta 11 Pre-fixing guide member 12 Secondary transfer device 13 Fixing device 13a Fixing roller 13b Pressure roller 15 First support roller 16 Second support Roller 17 Primary transfer roller 17M Primary transfer roller for magenta 17K Primary transfer roller for black 18 Third support roller 19 Fourth support roller 26 Paper feed cassette 27 Paper feed roller 28 Registration roller pair 29a First paper feed path 29b Second paper feed Path 30 Conveying roller pair 31 Transfer exit guide member 39 Discharge roller 40 Discharge tray 50 Paper feed table 60C Cyan transfer nip 60K Black transfer nip 60M Magenta transfer nip 60 Primary transfer nip 100 Printer 151 Optical sensor 152 Transfer nip downstream optical sensor 152Y Yellow transfer nip downstream optical sensor 152C Cyan transfer nip Downstream optical sensor 152M Magenta transfer nip downstream optical sensor 153 Sensor facing member 200 Tandem type image forming unit 500 Control unit L Laser light P Paper Sm Magenta gradation pattern image Sy Yellow gradation pattern image Vb Development bias Vd Charging potential VL Exposure part potential ΔV Potential difference

Japanese Patent Laid-Open No. 7-253694 JP-A-10-153909 JP 2010-020008 A

Claims (7)

A plurality of image forming means for forming toner images on the surfaces of a plurality of latent image carriers that move on the surface based on image information;
A plurality of transfer means for respectively transferring the toner image on the latent image carrier to an intermediate transfer member that moves on the surface;
An image forming apparatus comprising: a recording medium transfer unit that transfers a toner image on the intermediate transfer member to a recording medium;
Among the plurality of transfer means, the downstream transfer means having at least one other transfer means on the upstream side in the surface movement direction of the intermediate transfer body than the transfer means controls the transfer voltage by constant current control,
The upstream toner image transferred to the intermediate transfer member by the other transfer means upstream of the downstream transfer position at which the downstream transfer means performs the transfer before and after passing through the downstream transfer position. A toner adhesion amount variation detecting means for detecting a variation in the toner adhesion amount of the side toner image;
The downstream transfer unit changes a target current value in constant current control based on a detection result of the toner adhesion amount variation detection unit. The image forming apparatus according to claim 1.
A toner adhesion amount detection sensor for detecting a toner adhesion amount of a toner image on the surface of the intermediate transfer body on the downstream side in the surface movement direction of the intermediate transfer body with respect to each transfer position of the plurality of transfer units;
The toner adhesion amount fluctuation detecting means includes a toner adhesion amount of the upstream toner image detected by the toner adhesion amount detection sensor disposed upstream of the downstream transfer position, and a downstream side of the downstream transfer position. An image forming apparatus that calculates a difference between the toner adhesion amount of the upstream toner image detected by the toner adhesion amount detection sensor disposed in the image forming apparatus. The image forming apparatus according to claim 2.
An image forming apparatus comprising: a sensor shutter member that shields between the toner adhesion amount detection sensor and the intermediate transfer member. The image forming apparatus according to claim 2, wherein
The intermediate transfer member is a belt-like intermediate transfer belt,
An image forming apparatus comprising: a detection position counter member that contacts the back surface of the intermediate transfer belt at a position facing the toner adhesion amount detection sensor. The image forming apparatus according to claim 4.
The plurality of transfer units each include a transfer member facing the latent image carrier with the intermediate transfer belt interposed therebetween,
The transfer member is configured to be able to contact and separate from the intermediate transfer belt,
2. The image forming apparatus according to claim 1, wherein the detection position counter member is displaced in synchronization with the contact / separation operation of the transfer member. The image forming apparatus according to claim 4, wherein
The plurality of transfer units each include a transfer member facing the latent image carrier with the intermediate transfer belt interposed therebetween,
An image forming apparatus, wherein an electric resistance of the detection position opposing member is higher than an electric resistance of the transfer member. The image forming apparatus according to any one of claims 4 to 6,
The image forming apparatus according to claim 1, wherein the detection position opposing member has a roller shape.

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